The present invention generally relates to a central processing unit equipped with an extended storage unit corresponding to a highspeed access storage unit provided independently from a main storage unit. More particularly, the present invention is directed to a data transfer unit suitable for improving throughput of a data transfer effected between an input/output unit and the extended storage unit in the central processing unit to which the input/output unit capable of transferring the data having a large capacity at a high speed is connected.
There is a conventional data processing system employing the page storage unit, as described in U.S. Pat. No. 4,476,524, which is similar to the above-described central processing system equipped with the memory unit directly accessible from the instruction processor and the extended storage unit for executing the data transfer operation between this memory unit and the extended storage unit.
In accordance with the above-described conventional data processing system, the extended storage unit includes the data transfer path connected therewith the main storage unit. When the program is executed in the instruction processor, the data transfer operation is executed by executing the synchronous instruction of the instruction processor, or initiating the asynchronous channel program. The data transfer operation effected by the asynchronous channel program in the instruction processor is performed by providing the data transfer buffers on the main storage unit and by initiating the channel program with employment of the channel command word used for executing the data transfer operation between the main storage unit and the extended storage unit.
For instance, the read operation from the input/output unit to the extended storage unit with employment of the channel program is executed by combining the below-mentioned two channel command words in the above-described prior art:
(1) The data which has been transferred from the input/output unit to the channel unit is transferred to the data transfer buffer provided in the main storage unit by executing the read command of the input/output unit.
(2) The data stored in the data transfer buffer is transferred to the extended storage unit in response to the data transfer command from the main storage unit to the extended storage unit.
The data write operation is carried out in a similar manner to that of the above-explained data read operation. That is to say, after the data transfer operation has been executed from the extended storage unit to the main storage unit, the data is transferred from the main storage unit to the input/output unit.
The above-described conventional data processing system has such a problem that the data transfer operation cannot be effectively performed between the extended storage unit and the input/output unit for transferring the data having a large capacity at a high transfer speed. In other words, according to the prior art data processing system, since the main storage unit is used as the data transfer buffer, the following data transfer methods must be employed. Namely, the data transfer with a small capacity is repeatedly performed many times by preparing the data transfer buffer with a relatively small capacity and utilizing a large number of channel command words, or the data transfer with a large capacity is repeatedly carried out at a few time by preparing the data transfer buffer with a large capacity and utilizing a relatively small number of channel command words. Otherwise, the intermediate data transfer method must be employed in view of the capacity of the data transfer operation.
However, in the above-described method for preparing the data transfer buffer with such a small capacity on the main storage unit, the process time is required in that the channel command word is read and analyzed by the channel unit, and furthermore this analyzed command is transferred to the input/output unit. Since this process time becomes such a lengthy time that is not negligible, as compared with the transfer time, the overhead operation by the channel unit becomes great. Accordingly, it is impossible to achieve the data transfer operation with the high throughput at the high data transfer speed. Furthermore, according to another conventional method for preparing the data transfer buffer with such a large capacity on the main storage unit, although the overhead operation of the channel unit due to the analysis time for the channel command word becomes small, since the main storage unit is occupied with the data transfer buffer having the large capacity, there are many page faults. These pages are required to execute the program by the instruction processor which is under execution in conjunction with the input/output operation. As a consequence, the actual program executing time is lowered. Then, according to the above-described intermediate method with respect to the capacity of the data transfer buffer, since the above-described drawbacks belonging to both of the conventional methods are partially conducted to the intermediate method, both of the merits, i.e., improvements in the throughput of the data transfer and the program execution performance.
With the above-described reasons for the conventional data transfer operation to the extended storage unit, it is practically impossible to realize such an effective data transfer operation that both of the data transfer throughput and the high program execution speed can be achieved.